The increasing need for wireless networks and communication capabilities in outlying and geographically diverse locations has created greater demand for wireless systems. Many of the new carriers providing the infrastructure for such systems have focused their resources on building as many terrestrial cell stations as possible. As a result, the carriers expand their respective areas of coverage and consequently generate more revenue.
However, the buildout rate for the terrestrial base stations is typically slow and expensive, especially in mountainous areas and sparsely populated areas having few roads and minimal infrastructure buildout. In addition, in some the above-mentioned sparsely populated areas, a carrier's return on investment may not provide the incentive necessary for the carrier to build the necessary cell stations, thereby leaving these areas with either limited service or no wireless service at all. Further, many areas having a sufficient number of wireless communications base stations to handle calls during both off-peak and peak times cannot adequately handle temporarily large volumes of calls during sporting events or other special events that attract large crowds for just a few days.
In response to the above, airborne wireless systems have been proposed in which a wireless repeater mounted in an airplane, flying a predetermined flight pattern over a geographic area requiring wireless coverage, links calls from wireless phones within the covered geographic area to a terrestrial base station and other terrestrial infrastructure components. Because the airplane is capable of traversing geographic limitations and takes the place of the cell stations, such a system overcomes the above-mentioned problems.
Despite its many advantages, an airborne cellular system presents design and implementation considerations not present in the design and implementation of conventional terrestrial cellular systems. One primary consideration relates to maintaining a list of cell station call handoff candidates. Conventional cellular standards and protocols such as TIA/EIA 136, GSM and CDMA IS-95 provide for such handoff candidates. In terrestrial cellular systems, the handoff candidates are controlled in the system switch and are communicated to the handsets for power monitoring. The switch then makes hand-off decisions based on power measurement reports from the handsets. The number of hand-off candidates supported by the protocol is limited and typically does not vary with time. For example, the number of candidates is limited to 24 in the cellular TIA/EIA 136 protocol.
In an airborne cellular system, as the airplane circles in its flight pattern, communications beams radiated from the airplane antenna move relative to the ground thereby causing the system to perform call handoffs as beams rotate into and out of predetermined system areas of coverage. As an airborne cellular system covers a typically broad geographic area, each system beam potentially interacts with a large number of terrestrial cell sites. Therefore, it is likely that the total number of terrestrial cell sites that any given beam interacts with will exceed a number of handoff candidates supported by the given cellular protocol.
In addition, an airborne cellular system provides geographic coverage at the expense of large call capacity. Therefore, if an airborne cellular system were deployed in a predominantly low-density region that has pockets of high density, it would be desirable for a service provider to build terrestrial system cell stations in the high-density pockets and provide service to the remaining low-density areas with an airborne cellular system or systems. However, communications beams from the airborne cellular system would likely overlap with those of the terrestrial system cell stations. As the terrestrial system cell stations would typically have higher power than the communications beams of the airborne cellular system, system users would tend to gravitate to the terrestrial system cell stations in overlapping areas.
Users in areas not covered by terrestrial cell stations initially communicate through the airborne cellular system and can potentially switch over to the terrestrial system, as it may be desirable to hand off user calls from the airborne cellular system to the terrestrial system cell stations to reduce capacity on the airborne cellular system. As there are often hundreds of hundreds of terrestrial system cell stations, the airborne cellular system must generate a corresponding handoff candidate list that includes hundreds of cell station handoff candidates. Unfortunately, such a handoff candidate list is currently beyond the capability of standard cellular protocols and clearly a need exists for solutions to the foregoing problems.